Reef Chemistry Question of the Day #78 Sodium Silicate

[Randy Holmes-Farley]

How are you measuring alkalinity?

 

[Atu]

38/41º B

I know they look imposible, I don't believe them myself, and it falls to normal value too quickly also. At first I blamed it to taking the water sample from the same place that I dose, but the second time I changed that. I could do the experience again tomorrow. pH doesn't change when I dose for what is worth...
I could try to measure alkalinity using the pHmeter also, maybe something is interfering with the test kit..

 

[Atu]

I'm measuring with the salifert alkalinity titration kit. I do have some lab experience with titrations, and even if I'm doing it wrong (or the test kit is expired), I should be doing it wrong all the time, not only after dosing silicate. I already doubted myself because I can't explain this anyway and couldn't find any mention of it in any forum.

 

[Randy Holmes-Farley]

That silicate solution has a density of about 1.39 so you dosed about 7 grams to 400 gallons. 41 baume is equivalent to about 9% NaO, so 7 grams contains about 0.63 grams NaO which equals 0.016 moles or 16 mmoles. Diluted into 400 gallons, that’s only 0.011 meq/l or 0.03 dKH.

I’m not sure why you observed what you did.

 

[Atu]

Have you ever measured alkalinity just after dosing silicate? I wonder if I’m the only one with this results...
What I currently believe is that the sodium silicate is causing false readings in the test kit somehow because the alkalinity falls too fast for it to be a real value (we’re talking almost almost 1 dKh per hour) and stops when reaching the pre-dosing value or similar (to 0,5 dKh per day).
Tomorrow I’m gonna test again. Maybe I should test with a different method.

 

[Randy Holmes-Farley]

Have you ever measured alkalinity just after dosing silicate? I wonder if I’m the only one with this results...
What I currently believe is that the sodium silicate is causing false readings in the test kit somehow because the alkalinity falls too fast for it to be a real value (we’re talking almost almost 1 dKh per hour) and stops when reaching the pre-dosing value or similar (to 0,5 dKh per day).
Tomorrow I’m gonna test again. Maybe I should test with a different method.

I'd be highly surprised if there was a problem with an alk titration at the silicate levels you are encountering. I can't see how it could be an issue chemically (an alk titration is an incredibly simple test) and some folks naturally have silica far higher than your max after dosing.

You can do the alk titration by pH meter if you want to check it:

A DIY Alkalinity Test: By Randy Holmes-Farley
http://www.reefedition.com/a-diy-alkalinity-test-by-randy-holmes-farley/

 

[Atu]

Well, I found the problem, and unsurprisingly it was me.
I was using the same syringe for measuring the sodium silicate and, after extensive rinsing, taking the water sample for measuring alkalinity. The first time I though this was the problem, so I cleaned the syringe a lot (the rise in alkalinity the second time was lower consequently). Apparently rinsing it over 15 times with tank water wasn't enough, I just retested utilizing 2 different syringes and found no change in alkalinity before and after dosing. Retested also just to be certain.
The sodium silicate is really viscous and apparently not easy to remove from a syringe.
I started witch hunting after I lost a blastomussa to RTS...

Thanks for you help Randy, not only this time but for all the times I've read your posts and articles. I really felt like talking to someone famous after years of reading forums without writing. Hopefully this thread will make me start posting.

 

[Randy Holmes-Farley]

You're welcome.

And I'm glad the issue got you posting!

 

[Atu]

Randy, could long term dosing with few water changes have a significant effect on sodium levels?

 

[Randy Holmes-Farley]

Randy, could long term dosing with few water changes have a significant effect on sodium levels?

No. You are dosing a tiny amount relative to the normal level of 10,800 ppm sodium.

Also, at fixed salinity (which I presume you control), sodium can only rise if the other main cations (calcium, magnesium and potassium) fall. If those are all normal or higher, sodium cannot rise noticeably no matter how much sodium you dose.

 

[Atu]

Thanks!
I knew it was a small relative amount, but wasn't completely sure if longterm wouldn't affect the balance of ions. I do control for all you mentioned, so I'm at ease now.
By the way, either carbon dosing, silicate additions, or both, have contributed to a huge, and I mean HUGE, sponge. I thought they were a lot of separate sponges, but it turns out they are all connected like a web. It grows so fast it gets pruned regularly when it reaches areas of high flow.

 

[Randy Holmes-Farley]

I can imagine both contributing to the sponge growth.